# Helium Variation Across Two Solar Cycles Reveals A Speed-Dependent Phase Lag

## TL;DR

This study analyzes 22 years of solar wind data to reveal that the phase lag between helium abundance and sunspot activity depends on solar wind speed, indicating a speed-dependent depletion mechanism during solar wind formation.

## Contribution

It demonstrates for the first time that the phase delay between helium abundance and sunspot number varies with solar wind speed over a full solar cycle.

## Key findings

- Phase delay between $A_{He}$ and SSN increases with solar wind speed.
- Correcting for the lag aligns helium abundance across different solar cycle phases.
- Helium depletion mechanism is linked to solar wind speed during formation.

## Abstract

We study the relationship between solar wind helium to hydrogen abundance ratio ($A_\mathrm{He}$), solar wind speed ($v_\mathrm{SW}$), and sunspot number (SSN) over solar cycles 23 and 24. This is the first full 22-year Hale cycle measured with the Wind spacecraft covering a full cycle of the solar dynamo with two polarity reversals. While previous studies have established a strong correlation between $A_\mathrm{He}$ and SSN, we show that the phase delay between $A_\mathrm{He}$ and SSN is a monotonic increasing function of $v_\mathrm{SW}$. Correcting for this lag, $A_\mathrm{He}$ returns to the same value at a given SSN over all rising and falling phases and across solar wind speeds. We infer that this speed-dependent lag is a consequence of the mechanism that depletes slow wind $A_\mathrm{He}$ from its fast wind value during solar wind formation.